1. Technical Field
The present invention relates to a signal maintenance and management technology in a Wavelength Division Multiplexed(WDM) transmission network. Particularly, the present invention relates to an optical channel: dispersion compensating and monitoring apparatus when an optical signal transmits from an optical transmitter to an optical receiver through an optical fiber.
And the present invention relates to a WDM optical amplifier comprising the optical channel dispersion compensating and monitoring apparatus, for a long distance.
2. Background
A WDM method is widely used for maximizing data transmission capacity by multiplexing optical signal having a plurality of wavelengths different from each other, on an optical fiber. In the WDM method where the optical signal having several channels is commonly existing, it is very important to figure out each channel state for an efficient maintenance and management of the transmission network, thereby it is essential to monitor the optical channel.
Additionally, in order to increase the data transmission capacity in the WDM method, the number of the optical channel should be increased or transmission speed of each optical channel should be improved. In case of improving the transmission speed of the optical channel, it may happen a serious signal distortion by the optical fiber dispersion as a frequency bandwidth of each optical signal becomes widen. Accordingly, it has been studied for establishing the optical channel dispersion compensating and monitoring technology in the bulk WDM transmission system.
When the optical signal is transmitted through the optical fiber, the transmission speed becomes different according to the optical wavelength. The dispersion is caused by the different transmission speed in the optical fiber. In the dispersion compensating technology, optical element(for example, a dispersion compensating optical fiber, a dispersion compensating filter, etc.) is used for compensating the optical signal distortion generated by the optical fiber dispersion, and thereby makes overall dispersion value to xe2x80x9c0xe2x80x9d by having a dispersion value opposite to the transmission optical fiber dispersion value.
FIG. 1 shows a configuration diagram of a common dispersion compensating device. An optical signal outputted from an optical transmitting unit 11, passes through a Single Mode Fiber(SMF) 12. When the optical signal passes through the SMF, the signal is distorted. A compensated optical signal is detected in an optical receiving unit 14 after passing through a dispersion compensating optical element 13. A location of the SMF 12 and the dispersion compensating optical element 13 may be changed.
FIG. 2 shows a configuration diagram of a common optical channel monitoring device. The optical channel monitoring technology is used for obtaining information of each channel optical signal, such as optical signal power in the WDM system where optical signals having several channels are multiplexed. Accordingly, the technology is very important when improving the optical amplifier capacity as well as network operating, managing and control in the WDM system.
A portion of WDM optical signal 21 to be monitored is tapped 22 and then the tapped signal is applied to an optical channel monitoring circuit 23. Here, the tapped signal is a little amount which does not affect a signal transmission process. The optical channel monitoring circuit 23 may have information 24 of each channel from the tapped optical signal. The information 24 of each channel indicates an optical power of each channel as an electric signal.
Now, referring to FIGS. 3 to 4 the configuration and the operation of the conventional dispersion compensating technology is explained.
The dispersion compensating optical element 13 of FIG. 1 can be a Dispersion Compensating Fiber(DCF), a Chirped Fiber Bragg Grating(Chirped FBG), etc.
FIG. 3 shows a schematic diagram of a conventional dispersion compensating method using the DCF. It is composed of an optical transmitting unit 31, an SMF 34, a DCF 36 and an optical receiving unit 38. Wavelength elements(xcexLxcx9cxcexH) are limited in a given pulse width before being transmitted, referring to reference No. 33. But the optical signals of the transmitting unit 31 have a widening pulse width, caused by a dispersion effect after the optical signals are transmitted through the SMF 34(referring to No. 35). Here, an SMF dispersion coefficient(DSMF(ps/nm/km)) has a positive value. The DCF 36 compensates the dispersed optical signal 35 with a negative dispersion value(DDCF(ps/nm/km less than 0)) and applies the compensated optical signal pulse 37(same as the pulse 33) to the optical receiving unit 38. In other words, LSMF*DSMF+LDCF*DDCF≈0). The LSMF is a length of the SMF. The LDCF is a length of the DCF.
Therefore, it is capable to obtain the optical signal without distortion in the optical receiving unit 38 by setting a sum of the dispersion coefficients of the SMF 34 and the DCF 36 to be about xe2x80x9c0xe2x80x9d.
FIG. 4 shows a schematic diagram of a conventional dispersion compensating method using a Chirped FBG element.
A dispersion compensating device 41 is composed of a Chirped FBG element 42 for compensating channel dispersion and a circulator 43. When making gratings on an optical fiber, the chirped FBG element 42 is manufactured to have several different gratings to regulate wavelength path to be long or short. For example, xcexL is for a long reflecting path, xcexO is for a middle path, and xcexH is for a short path. For regulating the dispersion generated in the FBG element by managing the reflective paths according to the wavelengths, a dispersion value of the Chirped FBG element is a value opposite to a dispersion value of the SMF, thereby it is capable of compensating the dispersion distortion. A sum of the dispersion value between the chirped FBG and the SMF is about xe2x80x9c0xe2x80x9d. Because of using the gratings, the compensated optical signal of the Chirped FBG element is a reflective wave. Accordingly, a directional optical element such as the circulator should be used for transmitting the compensated optical signal to the optical receiver.
A signal incoming to first port of the circulator 43 is a distorted optical signal having a channel(xcexO) after passed through the SMF. Reference No. 45-1 is a pulse of the optical signal of the channel(xcexO) in a time zone. And reference No. 45-2 is a spectrum of the WDM optical signal in a frequency zone.
The distorted optical signal is outputted from second port of the circulator 43, passes through several reflective paths at each wavelength by the Chirped FBG element 42 to compensate the dispersion and then is applied to the optical receiver via third port of the circulator 43. The optical signals except wavelength(xcexLxcx9cxcexH) in a dispersion compensating bandwidth having wavelength(xcexxe2x88x921xcx9cxcex1) is passed therethrough, regardless of the FBG grating. A reference No. 47 is output signal spectrums.
Nowadays, the DCF is widely used. However, it is expected that the FBG element will be highly positioned in cost, size, and less error views according to fast development. Recently, the dispersion compensating device having an FBG element is provided for commercial usage.
Now, the configuration and the operation of the conventional optical channel monitoring method will be explained, referring to FIGS. 5 to 6.
FIG. 5 shows a schematic diagram of a conventional optical channel monitoring technology using a spectral element. A portion of WDM optical signal 51 to be monitored is tapped 52 and then the tapped signal is applied to a spectral element 53 which is different from the spectral element (for example, the FBG element in FIG. 4) for compensating dispersion. By using the spectral element 53, the tapped signal is divided into each channel wavelength(xcex1xcx9cxcexN).
Each of channel wavelengths(xcex1xcx9cxcexN) Is converted to electric signals(V1xcx9cVN) through corresponding Optical/Electric(O/E) converters, respectively. By using the electric signal, it is capable to measure a signal power of each optical channel. The spectral element is an optical filter, like an Arrayed Waveguide Grating element or a Bulk Grating element.
FIG. 6 shows a schematic diagram of a conventional optical channel monitoring device using a tone frequency.
Each of the channel wavelengths(xcex1xcx9cxcexN) (information channel having several GHz) which is still less than dithered frequency is dithered to a certain frequency(f1xcx9cfN) (in other words, tone frequency having several tens KHz) 60. Each of the channel wavelengths(xcex1xcx9cxcexN) of each optical transmitting unit(TX1(xcex1)xcx9cTXN(xcexN)) is multiplexed in an optical multiplexer 61 and then transmitted. The tapped WDM optical signal 63 is applied to an O/E converting and electric filtering unit 64 to monitor the channels and is outputted as electric signals(V1xcx9cVN) corresponding to each tone frequencies(f1xcx9cfN). The electric signals(V1xcx9cVN) indicates an optical power information of each of the channels (xcex1xcx9cxcexN) and therefore may check channels. The above method using the tone frequency is applicable in a Lucent WDM transmission device.
Because the conventional optical channel dispersion compensating and monitoring technologies are different in functions and size of the devices which are not small, each unit is separately implemented and installed on the WDM transmission device.
Accordingly, in order to solve the problems in the prior art it is first object of the present invention to provide an optical channel dispersion compensating and monitoring apparatus, the apparatus may compensate a dispersion of the optical signal in a reflective wave shape and convert a portion of the compensated optical signal(an optical power value) to electric signal respectively.
And second object of the present invention provides a WDM optical amplifier having the optical channel dispersion compensating and monitoring apparatus which is enough to small to be installed inside of the WDM amplifier, therefore capable of controlling noise and having gain flatness of the overall optical amplifier.
In an embodiment to achieve first object of the present invention, Wavelength Division Multiplexing (WDM) optical channel dispersion compensating and monitoring apparatus when transmitting optical signal in a WDM system, compensates the optical signal distorted in an optical fiber at each channel and monitors each channel by using a portion of the compensated optical signal.
In another embodiment to achieve first object of the present invention, WDM optical channel dispersion compensating and monitoring apparatus in a WDM system, groups a plurality of distorted optical signals according to capacity of wideband chirped FBG elements, compensates the grouped distorted optical signals respectively and monitors each channel by using a portion of the compensated optical signal.
In other embodiment to achieve second object of the present invention, an optical amplifier in a WDM system comprises a WDM optical channel dispersion compensating and monitoring apparatus for compensating the optical signal distorted in an optical fiber at each channel and monitoring each channel by using a portion of the compensated optical signal, a controller for controlling output characteristics of the amplifier according to the channel monitored by the apparatus, and a WDM optical amplifying unit for amplifying the compensated optical signal.